(1) Field of the Invention
The present invention relates to a combined in-plane shear and biaxial tension or compression loading apparatus, having hydraulic or pneumatic force controls to independently control the loading along, varying axis of a test specimen, for testing mechanical properties of metals, plastics, composites, woods, fabrics, elastomers, and other materials as the test specimen.
(2) Description of the Prior Art
It is known in the art that pressurized fabric tubes; pressure-stabilized beams (also known as air beams) and air-inflated structures are practical fixtures for lightweight and rapidly deployable structures such as temporary shelters, tents, temporary bridges and space structures. Presently, plain-woven fabrics have been utilized in air-inflated structures. As such, design optimization of an air-inflated structure depends on a thorough understanding of woven fabric mechanics.
Furthermore, the advent of structural fiber materials and weaving/braiding technologies has improved the load carrying capacity of pressurized fabric structures. Accordingly, there has been increasing interest in modeling the mechanical behavior of woven fabrics. However, this class of materials has complex microstructures that lead to complex mechanical responses. In particular, the mechanical characteristics of plain-woven fabrics used in inflated structures exhibit high non-linearity with dependence on the internal pressure and contact interactions within the woven fabric.
Therefore, there is a need for a testing apparatus, which allows the measurement of the elastic and shear moduli for air beams since induced inflation pressure creates a biaxial loading in fabric. To measure the shear moduli of the fabric, an in-plane shear loading is needed. Specifically, there is a need for a testing apparatus capable of applying combined in-plane shear load and biaxial tensile load. There is a further need that the test apparatus be capable of loading non-orthogonal composite or fabric materials with equi-biaxial or non-equi biaxial loading.
Biaxial testing apparatuses or in-plane shear testing apparatuses are known in the art; however, none of the apparatuses have a combined feature of in-plane shear and compression/tension testing capabilities. Furthermore, none of the apparatuses of the prior art are capable of applying a non-orthogonal biaxial loading. Prior art methods typically employ two or more separate actuators in complex test fixtures and/or pressurization techniques to apply a biaxial load to a test specimen. A disadvantage of these methods is the need for two or more loading devices and the high cost of the equipment. A review of the following references reveals the disadvantages of the prior art.
In Clay, (U.S. Pat. No. 5,905,205), an in-plane biaxial test apparatus is disclosed which comprises linkages to transfer a load to the orthogonal direction of the loading. In the reference, a rhombus-shaped four-bar linkage is attached at one vertex to a fixed attachment point and a uniaxial tensile force is applied to the opposite vertex. The test specimen is placed inside the linkage and is attached to the linkage by load transfer members connected at one end to the links of the linkage and at their other end to grips holding the test specimen. Load transfer members parallel to the applied uniaxial tensile force are attached to test specimen grips adjacent to the link attachment points of the load transfer members and perpendicular load transfer members are attached to test specimen grips opposite their link attachment points. Application of a uniaxial tensile force produces a biaxial tensile force in the test specimen. A disadvantage of the test apparatus is that it is not capable of applying in-plane shear to the test specimen. Another disadvantage is that the biaxial loading is limited to an orthogonal configuration.
In Tucchio, (U.S. Pat. No. 5,448,918), an apparatus with an X-shape is disclosed which is only used for compression load. The compression testing device is formed by two modified beams joined to form an X-shape with the support structure, such as webs and upper flanges, removed in the region of the X intersection, thereby leaving a rectangular opening. The rectangular opening has dimensions slightly greater than the widths of the beams and is open from the upper surfaces downward to the lower surfaces, which are joined together forming an X-configuration. This configuration has a flexing characteristic in the direction perpendicular to the plane of the joined beams. A test specimen support plate is attached to the underside of one of the upper surfaces and is located so as to slide below the opposing upper surface during flexing of the X-beam assembly. Each beam is supported by a roller pin. Additional roller pins are located on the specimen support plate between each beam upper flange and a specimen to be tested. A disadvantage of this apparatus is that these roller pins prevent any torsional load from reaching the test specimen.
In Ward et al., (U.S. Pat. No. 5,279,166), an apparatus for self-alignment of a biaxial loading device is disclosed. The apparatus is for testing the strength of specimens while maintaining a constant specimen centroid during the loading operation. The apparatus consists of a load frame and two load assemblies for imparting two independent perpendicular forces upon a test specimen. The test specimen centroid is maintained by providing elements for linear motion of the load frame relative to a fixed crosshead, and by alignment and linear motion elements of one load assembly relative to the load frame.
In Mathiak et al., (U.S. Pat. No. 5,144,844), a cruciform planar specimen for biaxial material testing is disclosed. A flat cross-shaped test piece is made of sheet metal for biaxially testing. This test piece includes a central region that defines an area of measurement. Four arms for applying loads to the central region extend from the central region along orthogonal axes. Each arm has one end integral with the central region and an opposite end with an end part for connection to a test device for the application of a test load. Tensile stresses can thus be applied to the central region along first and second orthogonal coordinate axes of the central measurement region. Slots in the load applying arms extend along the arms parallel to the first and second coordinate axes from the end part as far as and up to the area of measurement.
In Vanderlakis et al., (U.S. Pat. No. 4,885,941), an apparatus for compressive loading of geo-materials is disclosed. The test apparatus for geomaterial (soil, etc.) samples is designed to allow free shear band formation and provide measurements of the stress displacement characteristics of the failure zone. A geomaterial sample formed into a specimen comprising a right rectangular prism is surrounded by a thin rubber membrane and is supported by walls along two parallel faces. An axial load is kinematically applied by a plate that is guided to prevent any tilt or eccentricity, while a bottom support plate for the specimen is horizontally guided by a linear bearing that is substantially friction free. The assembly of the specimen and its supports is placed in a conventional tri-axial cell in a loading frame so that an axial load can be applied to one end of the specimen and reacted against the bottom plate. Internal loaded load cells allow for measurement of the axial force as well as friction along the side walls. Displacement transducers monitor the axial and lateral displacements of the specimen and the horizontal movement of the bottom plate.
In Holt, (U.S. Pat. No. 4,192,194), an apparatus for biaxially loading a specimen through pressurizing the inside surface of a cylinder is disclosed. A thin-wall tube specimen is biaxially tested for stress analysis by applying compressive axial stress and either internal surface pressure or external surface pressure to the specimen. Torsion is not required. The sample is positioned between platens, which are assembled inside a pressure collet. Axial compressive stress is applied through the platens to the specimen, and hydraulic pressure is applied through the assembly to the internal and external cylindrical surfaces of the specimen. The disadvantages of this art include the requirement of cylindrical shape of the specimen and the high cost and added equipment of pressurization.
In Lynch (U.S. Pat. No. 3,776,028), an apparatus requiring three independent loading mechanisms is disclosed. A three-axis, adjustable loading structure is provided for test equipment wherein it is desired to exert pressure against the structure, which is to be tested. The device of the present invention is provided with three electric drives whereby the wall angle, horizontal position, and vertical position of the test device can be positioned.
None of the above-mentioned devices and apparatuses of the cited references are capable of combining the in-plane and compression/tension loading of a test specimen while using only one loading system.
In the commonly-assigned reference, Cavallaro et al. (U.S. Pat. No. 6,860,156), a test apparatus is disclosed. The apparatus is capable of simultaneously or independently applying in-plane biaxial and shear loading to a test specimen. However, in the apparatus, the loading is applied to the test specimen by way of equal biaxial extension (or contraction).
An improvement for some material testing is where the actual applied load, not the displacement, can be controlled and applied to the test specimen. Also, in creep testing material testing of composites, anisotropic and fabrics, the tension or compression forces on the test specimen could be kept constant. By controlled loading, the axes in the plane of the specimen could be subjected to varying tension or compression, (i.e. one axis having a different loading mode than another axis). The apparatus could be easily accommodated in a conventional material testing machine to be cost effective.